1. Technical Field of the Invention
This invention relates to telecommunication systems. More particularly, and not by way of limitation, the invention is directed to a communications node and method for providing control functions in a telecommunications network utilizing the Session Initiation Protocol (SIP).
2. Description of Related Art
Wireless telecommunication networks are evolving from second generation (2G) circuit-switched networks to third generation (3G) packet-switched networks. A reference architecture for a 3G wireless network is being developed by the Third Generation Partnership Project (3GPP). The 3GPP network architecture uses the Session Initiation Protocol (SIP) developed by the Internet Engineering Task Force (IETF) for call setup signaling. Media is then transported through an existing IP network. The SIP standard is described in RFC 2543 which is hereby incorporated in its entirety by reference herein.
In the 3GPP network, control signaling, often referred to as the “control plane”, is kept separate from the payload or media, often referred to as the “user plane”. When a mobile terminal (MT) is first activated, it registers its existence on a sub-network utilizing SIP call-control signaling through a Call State Control Function (CSCF). The SIP standard is a functional standard and, therefore, does not dictate a specific implementation for the CSCF. Functionally, the CSCF is broken down into a Proxy CSCF (P-CSCF), an Interrogating CSCF (I-CSCF), and a Serving CSCF (S-CSCF). The P-CSCF is the node that the MT directly communicates with, and is the MT's entry point into the SIP network. When the MT first registers, the P-CSCF determines the MT's home network using the domain name in the SIP REGISTER message and a Domain Name Server (DNS). The P-CSCF performs authentication and verification with the specified home network, performs some policy control in terms of determining what the MT is authorized to do, and performs a simple routing function based on a DNS lookup to route the REGISTER message to an I-CSCF in the home network.
The I-CSCF is the entry point into the home network, and serves as a boundary between the home network and a visited network into which the MT may roam. The I-CSCF also queries the MT's Home Subscriber Server (HSS) to identify an S-CSCF for the MT, and then routes the signaling to the S-CSCF. During registration, when the I-CSCF queries the HSS, the HSS determines that the MT does not have an S-CSCF assigned, and instructs the I-CSCF to select an S-CSCF from a plurality of S-CSCFs in the network. The I-CSCF selects one of the S-CSCFs in the network and assigns the MT to the S-CSCF. As long as the registration is valid, that S-CSCF is the MT's S-CSCF. The S-CSCF performs call setup and other telephony services for the MT. Once the MT is registered, the S-CSCF informs the HSS that the S-CSCF is now serving the MT. When calls come in for the MT, and the I-CSCF queries the HSS, the HSS responds with the identity of the assigned S-CSCF.
An originating user need not specify the exact destination address associated with the destination user. The 3GPP network uses aliases associated with particular users to automatically determine the identity of their registered terminals or devices, and to automatically format and deliver communications with the registered devices over an existing IP network. Thus, the 3GPP network architecture provides a centralized and independent communication control mechanism. For a registered user, the 3GPP network and associated elements keep track of the user's exact location and the identity of the user's registered terminal, and accordingly route and enable communication with that registered user over the existing IP network.
In addition to the three types of CSCFs, there are other types of control functions in the SIP network such as Media Resource Control Functions (MRCFs) and Border Gateway Control Functions (BGCFs). An MRCF is used for setting up and controlling conference calls. When two types of user equipment are to be joined in a conference call, and they do not have a common codec, the MRCF handles the signaling to set up digital signal processing hardware for media transcoding, and to start the codecs. The MRCF manages the conference, connects the legs of the call, and so on. A BGCF is used when non-SIP entities are to be joined in a session in the SIP-controlled IP network.
In existing implementation architectures, each control function is implemented independently. That is, the P-CSCF, I-CSCF, S-CSCF, MRCF, BGCF, and other control functions are separate nodes in the SIP network. From the development point of view, this is an inefficient process since there is a large amount of duplicated effort when designing these independent control functions. It would be advantageous to have a more efficient development methodology and control-function architecture. The present invention provides such an architecture and method.